ABG has developed a proprietary calculation method in accordance with the BRE 470 health & safety guidance, with the objective of improving safety via the construction of reliable and economic working platform designs. As part of this methodology, geosynthetics are typically included beneath or within a granular platform in order to improve its structural stability and minimise deformations during trafficking and loading.
The ABG range of geosynthetics typically used to construct working platform designs are Terrex geotextiles and a range of geogrids. Geotextiles are primarily used as a protective separation layer to prevent the loss of fines and contamination of the platform material and to provide a secondary degree of reinforcement/stabilisation. Geogrids are specifically required to provide reinforcement and stabilisation of the granular fill, providing a lateral restraint via friction and interlock/confinement. As a rule of thumb, a layer of geogrid is equivalent in cost to 100mm of granular fill and it is generally safe and economically justifiable to incorporate a grid when the platform depth can be reduced by 100mm or more.
For stabilising haul roads and site access routes to working platform areas with soft subgrades, ABG can provide design assistance to strengthen the in-situ soils in accordance with the foundation classes stated in clause 2.7, Note 2 of Design Manual for Roads and Bridges (DMRB) document CD 225.
Geosynthetic cellular confining systems such as Abweb may also be used to provide additional stabilisation of working platforms and to protect tree roots that are to be preserved alongside the installation of temporary work areas (Abweb TRP).
Geogrids
ABG provide a range of punched and drawn polypropylene geogrids with a typical design life in excess of 100 years and with aperture sizes to maximise stone interlock within the grids. They provide reinforcement of loading platforms in areas of weak compressible or variable sub-soils. In addition, by improving interlock/confinement, the geosynthetic acts to increase the load spread angle via increased shear resistance within the platform. Where a reclaimed on site fill is to be used, the geogrid can be selected to suit the specified maximum particle size.
Geotextiles
The Terrex geotextile layer primarily acts as a separation layer to prevent contamination and loss of fines content and to allow efficient drainage of the platform. Since it also acts in tension under loading, especially in soft soil conditions and often in combination with a geogrid, it provides additional restraint against the outward movement of the fill material and to confine the lowest layers of fill when they are compacted. Geotextiles used for working platforms must have sufficient puncture resistance to minimise damage from the gravel platform and in order to maintain exclusion of contaminants and retain tensile capacity. Terrex SNW is available where high resistance to puncture is required.
Design approach
Working platform design is carried out by our experienced geotechnical engineers using ABG’s in-house empirical load spread method and following the HSE guidance as detailed in BRE document ‘BR470 – Working platforms for Tracked Plant’. The layer configuration specified will depend on the subgrade strength, the platform materials and the required construction traffic loads, and our designs seek to achieve a balance between platform safety with adequate bearing capacity, whilst still being economic and straightforward to construct. In our design proposal we consider the site plan and the possibilities to re-use existing materials as a carbon saving alternative.
Drainage of the platform is also key and a poorly drained platform will deteriorate more rapidly. ABG has over 30 years’ experience in providing environmental drainage and where compressible subgrade soils require protection from water ingress, ABG can suggest a suitable drainage geocomposite option to drain excess rainwater to nearby channels. A geotextile may also be specified to treat platforms that are close to, or below the ground water table. ABG’s technical service includes initial budget cost estimates and indicative thickness calculations, through to full PI backed designs available on request.
Input parameters required for design
ABG’s design approach promotes H&S first. A properly designed and installed working platform enhances the efficient working of a site, as well as ensuring a safe environment. The design brief should be developed as required for any other temporary works design, but ideally the following information is required:
- What is the subgrade formation like:
- Ground investigation report
- Cohesive or granular soil
- Undrained shear strength
- Angle of friction
- Ground water table
- What is the platform to be made of:
- Specific source of gravel from site, or gravel to be specified and delivered
- Site layout:
- Plan of the working platform area and access roads
- Existing services survey (above and below ground)
- Existing structures survey (below ground chambers, retaining walls etc)
- What equipment will be operating on the working platform:
- FPS certificate / plant data sheets (dimensions, configurations, weights etc)
- Crane track ground bearing pressures, outrigger or mast foot loads
Typical client profile
- Piling Contractors
- Foundation Engineers
- Civil Engineering Consultants
- Housebuilders
- Highways Engineers
- Rail Engineers
- Temporary Works Managers
- Main Contractors
